Re: [PATCH RFC 12/19] drm/bridge: Add an ->atomic_check() hook

[Boris Brezillon <boris.brezillon@collabora.com>]

On Thu, 22 Aug 2019 03:12:07 +0300
Laurent Pinchart <laurent.pinchart@ideasonboard.com> wrote:

> Hi Boris,
> 
> Thank you for the patch.
> 
> On Thu, Aug 08, 2019 at 05:11:43PM +0200, Boris Brezillon wrote:
> > So that bridge drivers have a way to check/reject an atomic operation.
> > The drm_atomic_bridge_chain_check() (which is just a wrapper around
> > the ->atomic_check() hook) is called in place of
> > drm_bridge_chain_mode_fixup() (when ->atomic_check() is not implemented,
> > the core falls back to ->mode_fixup(), so the behavior should stay
> > the same for existing bridge drivers).
> > 
> > Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
> > ---
> >  drivers/gpu/drm/drm_atomic_helper.c | 11 +++---
> >  drivers/gpu/drm/drm_bridge.c        | 59 +++++++++++++++++++++++++++++
> >  include/drm/drm_bridge.h            | 23 +++++++++++
> >  3 files changed, 87 insertions(+), 6 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/drm_atomic_helper.c b/drivers/gpu/drm/drm_atomic_helper.c
> > index e76ec9648b6f..3fadde38ead7 100644
> > --- a/drivers/gpu/drm/drm_atomic_helper.c
> > +++ b/drivers/gpu/drm/drm_atomic_helper.c
> > @@ -445,12 +445,11 @@ mode_fixup(struct drm_atomic_state *state)
> >  		encoder = new_conn_state->best_encoder;
> >  		funcs = encoder->helper_private;
> >  
> > -		ret = drm_bridge_chain_mode_fixup(encoder,
> > -					&new_crtc_state->mode,
> > -					&new_crtc_state->adjusted_mode);
> > -		if (!ret) {
> > -			DRM_DEBUG_ATOMIC("Bridge fixup failed\n");
> > -			return -EINVAL;
> > +		ret = drm_atomic_bridge_chain_check(encoder, new_crtc_state,
> > +						    new_conn_state);
> > +		if (ret) {
> > +			DRM_DEBUG_ATOMIC("Bridge atomic check failed\n");
> > +			return ret;
> >  		}
> >  
> >  		if (funcs && funcs->atomic_check) {
> > diff --git a/drivers/gpu/drm/drm_bridge.c b/drivers/gpu/drm/drm_bridge.c
> > index 0528ca941855..dcad661daa74 100644
> > --- a/drivers/gpu/drm/drm_bridge.c
> > +++ b/drivers/gpu/drm/drm_bridge.c
> > @@ -583,6 +583,65 @@ void drm_atomic_bridge_chain_enable(struct drm_encoder *encoder,
> >  }
> >  EXPORT_SYMBOL(drm_atomic_bridge_chain_enable);
> >  
> > +static int drm_atomic_bridge_check(struct drm_bridge *bridge,
> > +				   struct drm_crtc_state *crtc_state,
> > +				   struct drm_connector_state *conn_state)
> > +{
> > +	if (bridge->funcs->atomic_check) {
> > +		struct drm_bridge_state *bridge_state;
> > +		int ret;
> > +
> > +		bridge_state = drm_atomic_get_new_bridge_state(crtc_state->state,
> > +							       bridge);
> > +		if (WARN_ON(!bridge_state))
> > +			return -EINVAL;
> > +
> > +		ret = bridge->funcs->atomic_check(bridge, bridge_state,
> > +						  crtc_state, conn_state);
> > +		if (ret)
> > +			return ret;
> > +	} else if (bridge->funcs->mode_fixup) {
> > +		if (!bridge->funcs->mode_fixup(bridge, &crtc_state->mode,
> > +					       &crtc_state->adjusted_mode))
> > +			return -EINVAL;
> > +	}
> > +
> > +	return 0;
> > +}
> > +
> > +/**
> > + * drm_atomic_bridge_chain_check() - Do an atomic check on the bridge chain
> > + * @encoder: encoder object
> > + * @crtc_state: new CRTC state
> > + * @conn_state: new connector state
> > + *
> > + * Calls &drm_bridge_funcs.atomic_check() (falls back on
> > + * &drm_bridge_funcs.mode_fixup()) op for all the bridges in the encoder chain,
> > + * starting from the last bridge to the first. These are called before calling
> > + * &drm_encoder_helper_funcs.atomic_check()
> > + *
> > + * RETURNS:
> > + * 0 on success, a negative error code on failure
> > + */
> > +int drm_atomic_bridge_chain_check(struct drm_encoder *encoder,
> > +				  struct drm_crtc_state *crtc_state,
> > +				  struct drm_connector_state *conn_state)
> > +{
> > +	struct drm_bridge *bridge;
> > +
> > +	list_for_each_entry_reverse(bridge, &encoder->bridge_chain,
> > +				    chain_node) {
> > +		int ret;
> > +
> > +		ret = drm_atomic_bridge_check(bridge, crtc_state, conn_state);
> > +		if (ret)
> > +			return ret;
> > +	}
> > +
> > +	return 0;
> > +}
> > +EXPORT_SYMBOL(drm_atomic_bridge_chain_check);
> > +
> >  /**
> >   * drm_atomic_helper_init_bridge_state() - Initializes a bridge state
> >   * @bridge this state is referring to
> > diff --git a/include/drm/drm_bridge.h b/include/drm/drm_bridge.h
> > index 9383b4e4b853..5d8fe3709bde 100644
> > --- a/include/drm/drm_bridge.h
> > +++ b/include/drm/drm_bridge.h
> > @@ -379,6 +379,26 @@ struct drm_bridge_funcs {
> >  	 */
> >  	void (*atomic_destroy_state)(struct drm_bridge *bridge,
> >  				     struct drm_bridge_state *state);
> > +
> > +	/**
> > +	 * @atomic_check:
> > +	 *
> > +	 * This method is responsible for checking bridge state correctness.
> > +	 * It can also check the state of the surrounding components in chain
> > +	 * to make sure the whole pipeline can work properly.  
> 
> As this is meant to replace .mode_fixup() you should document that here
> and in the .mode_fixup() operation.
> 
> There's also the additional question of what parameters can be modified.
> Is a bridge .atomic_check() allowed to modify the CRTC or connector
> state ? Of so, what are the restrictions on how those can be modified ?
> For instance .mode_fixup() can change the pixel clock but isn't allowed
> to modify the h/v active values. Those restrictions were not clearly
> documented for .mode_fixup() and have led to grey areas. I don't want to
> repeat the same mistake for bridges, we need to be very explicit here.

Any recommendation? I didn't face this problem myself, so I'd need
inputs from those who did to take a decision.

> 
> In the list of open questions, what if a bridge changes the pixel clock
> to a value it supports (as .mode_fixup() does), and then the previous
> bridge in the chain (the next one in traversal order for the
> drm_atomic_bridge_chain_check() function) also changes the pixel clock,
> to a value that the current bridge doesn't support ? I think we really
> need to think about the semantic of this operation.

Don't we have the exact same problem right now with the ->mode_fixup()
hook when bridges are chained? Not sure why it suddenly becomes a
priority to clarify that, but if we quickly settle on this new semantic,
I'm fine integrating those changes in my patchset.

> 
> As the goal of your series is to implement configuration of bus formats
> between bridges, shouldn't a bridge .atomic_check() also receive the
> state of the next bridge in the chain ?

I thought we had enough arguments passed to the atomic_check() hook and
decided to let the driver retrieve the next state. I can pass the next
bridge state if you prefer.

> 
> > +	 *
> > +	 * &drm_bridge_funcs.atomic_check() hooks are called in reverse
> > +	 * order (from the last to the first bridge).
> > +	 *
> > +	 * This method is optional.
> > +	 *
> > +	 * RETURNS:
> > +	 * zero if the check passed, a negative error code otherwise.
> > +	 */
> > +	int (*atomic_check)(struct drm_bridge *bridge,
> > +			    struct drm_bridge_state *bridge_state,
> > +			    struct drm_crtc_state *crtc_state,
> > +			    struct drm_connector_state *conn_state);
> >  };
> >  
> >  /**
> > @@ -479,6 +499,9 @@ void drm_bridge_chain_mode_set(struct drm_encoder *encoder,
> >  void drm_bridge_chain_pre_enable(struct drm_encoder *encoder);
> >  void drm_bridge_chain_enable(struct drm_encoder *encoder);
> >  
> > +int drm_atomic_bridge_chain_check(struct drm_encoder *encoder,
> > +				  struct drm_crtc_state *crtc_state,
> > +				  struct drm_connector_state *conn_state);
> >  void drm_atomic_bridge_chain_disable(struct drm_encoder *encoder,
> >  				     struct drm_atomic_state *state);
> >  void drm_atomic_bridge_chain_post_disable(struct drm_encoder *encoder,  
> 

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